One of my girls has lost weight. Enough so that I am starting to feel her spine. She is eating and no one is picking on her or preventing her from eating.What chronic diseases cause weight loss in mice? She has no tumors that I can see anyway. They eat the carnival hamster and mouse food.

She can easily have a tumor you can't see. She could have one of dozens of other issues saddly. You can try taking her to a vet to get on antibiotics as a attempt to see if she has an infection, or the vet could have other suggestions. It would run 100+ dollars depending on where you are located and your vet. You could also seperate her and try feeding better food. Carnival food is not very good quaility. Milkbones, dog kibble, labblock, oats, cheerios, bread, scrambled egg, and mealworms could possibly help. Most people here would cull her at this point. It's a hard thing to do, but helps to end their possible suffering.